The fake phone calls, some of which involve live callers, continued to crop up in Virginia, North Carolina and Florida, primarily among African-American voters, said Barbara Arnwine of the Lawyers Committee for Civil Rights Under Law. The group has mounted a counteroffensive of tens of thousands of calls reminding voters they can’t cast ballots over the phone.

“That is really dirty,” said Arnwine, who added that the callers’ identities remain a mystery. “It’s a very sophisticated operation and it’s very widespread, and it’s very troubling to us.”

…Voting-rights advocates pointed with particular concern to the tea party-linked True the Vote organization, which has pledged to dispatch thousands of monitors to polling places to guard against potential voter fraud. True the Vote President Catherine Engelbrecht rejected suggestions that the group would be overly aggressive or issue false challenges.